Mechanical strength of metals

ABSTRACT

A METHOD FOR IMPROVING THE MECHANICAL AND PHYSICAL PROPERTIES OF ALPHA IRON STEEL BAR AND ROD ARTICLES BY PLASTICALLY TWISTING THE ARTICLES WITHOUT DIMENSIONAL DEFORMATION BETWEEN ABOUT 200*C. AND ABOUT 400*C. THE STEEL BAR AND ROD ARTICLES ARE REQUIRED TO HAVE INTERSTITIAL SOLUTE ATOMS SELECTED FROM THE GROUP CONSISTING OF CARBON AND NITROGEN IN AN AMOUNT OF AT LEAST 0.02% CARBON AND AT LEAST 0.008% NITROGEN.

Vickers Hardness (Hv) Vickers Hardness (Hv) April 6, 1971 ISAO GOKYU3,573,999

MECHANICAL STRENGTH OF METALS Filed 001;. 26, 1966 '60 Outer SurfeeeInner Surface I 50 j l 40 Central Portion Inner Surface '20 OuterSurface I I0 *Central Portion Measured Positions IOO l WorkingTemperature (C) INVENTOR. IS A 0 G0 n Yo! United States Patent 3,573,999MECHANICAL STRENGTH 0F METALS Isao Gokyu, Musashinoshi, Tokyo, Japan,assignor to Nippon Kokan Kabushiki Kaisha, Tokyo, Japan Filed Oct. 26,1966, Ser. No. 594,348 Claims priority, application Japan, Oct. 30,1965, 40/ 66,312 Int. Cl. C21d 7/10 US. Cl. 148-42 Claims ABSTRACT OFTHE DISCLOSURE A method for improving the mechanical and physicalproperties of alpha iron steel bar and rod articles by plasticallytwisting the articles without dimensional deformation between about 200C. and about 400 C. The steel bar and rod articles are required to haveinterstitial solute atoms selected from the group consisting of carbonand nitrogen in an amount of at least 0.02% carbon and at least 0.008%nitrogen.

The present invention relates to a method of treating metals so as toimprove the mechanical strength thereof.

In particular, the method of the invention relates to the treatment ofsteel by warm-working thereof.

The expression warm working is intended to signify plastic working ofmetals and alloys at an elevated temperature which is substantiallyhigher than room temperature while being substantially lower than therecrystallization temperature thereof, so that the Warm-workingtemperature is within a warm temperature range.

It is known that the mechanical strength of steel can be increased bysubjecting it to plastic deformation, such as, for example, duringrolling or die drawing, with the temperature being in the neighborhoodof 300 0, although the precise reasons for this phenomenon have not, asyet, been completely explained. An important drawback of conventionalmethods of this type resides in the fact that there is a considerabledeformation, in the form of a reduction in the thickness orcross-sectional area of the material, which necessarily accompanies thepractice of the method. This disadvantage is so great that methods ofthis type have not become very popular with manufacturing industriesbecause of the possibility of unavoidable complications in production orof increased difficulties in maintaining the dimensional stability ofthe product.

Thus, it is a primary object of the present invention to provide amethod of increasing the strength of metals, without, however, givingrise to any substantial dimensional changes, alterations, or deformationthereof. Also, the present invention contemplates industrialapplications of the method which can be readily integrated into currentmass-production procedures. Other objects included in the scope of theinvention will be described in part or will be easily surmised by thoseWho are skilled in the art.

The inventor of the present invention came to realize, after many testsand investigations, that in order to achieve the same, or even better,results from the abovementioned warm-working, those types of plasticworking which are accompanied by dimensional changes or alterations arenot at all necessary. In fact, it was found that such relatively simpleprocedures as straightening, levelice ling, or twisting, which do notresult in any substantial dimensional change, but instead restoreflatness or straightness, can be satisfactorily used in the method ofthe invention provided that certain conditions are met and observed.Thus, it was established by hardness measurements and by the use of anelectron microscope as well as other pertinent mechanical andmetallurgical tests that there are four major factors involved in thestraightening process, of which two are controllable and the other twoare not controllable to the same extent.

The first of these factors is the temperature range at which the workingtakes place, this temperature range being from approximately 200 C. toapproximately 400 C., and preferably being in the neighborhood of 300 C.The second factor is the presence of interstitial solut atoms, such as,for example, carbon or nitrogen. The third factor is the nature of theworking which must be capable of generating a large number ofdislocations in the alpha iron matrix. The fourth factor is the state ofthe dislocations or dislocation configuration at the end of the plasticworking, this state of dislocations or dislocation configuration beingrequired to be such that an effective immobilization of dislocationsresults.

The invention is described in detail with reference to the accompanyingdrawings in which:

FIG. 1 is a diagram showing hardness measurements of a steel pipe priorto and after the application of one embodiment of a method according tothe present invention;

FIG. 2 is a schematic end view of the above-mentioned steel pipe, FIG. 2showing the locations where the hardness measurements were taken; and

FIG. 3 is a diagram showing hardness changes in alpha iron according todifferent temperatures of warm-working, this working taking placeaccording to another embodiment of a method of the present invention.

EMMPLE 1 A welding pipe, such as that shown in FIG. 2 having the weldedseam W, having a chemical composition of 0.09% carbon, a trace ofsilicon, 0.30% manganese, 0.030% phosphorus, 0.025% sulphur, and havingbeen shaped to predetermined dimensions according to which the pipe hasa diameter of 27.3 mm., a thickness of 2 mm., and a length of 1 m. by aconventional method, was straightened by passing a pair of obliquelevelling rolls at a temperature of 300 C. No measurable dimensionaldeformations occurred. The Vickers hardness, measured at the locations1, 2, 3 and 4 indicated in FIG. 2, where the welded seam W is shownsituated between the locations 2 and 3, for the inner, middle and outerportions of the pipe wall, clearly indicate an appreciablestrengthening, on the order of 25 in terms of the Vickers hardnessnumbers, as is indicated in-FIG. 1.

EXAMPLE 2 1 kg. of alpha iron of a composition listed below in Table lwas vacuum melted and cast into an ingot having a diameter of 60 mm. anda length of mm. The ingot was hot-rolled in a conventional manner toform a rod of a diameter of 6 mm., cooled to room temperature, thencold-drawn to a wire of 3 mm., diameter, and finally annealed in anevacuated atmosphere for 24 hours at 850 C.

3 TABLE 1 Element, percent:

Mn Trace The sample was divided into two portions. Specimens from one ofthe portions were twist-worked at room temperature, 100" C., 200 C., 300C., and 400 C. to surface strains of 4%, 13%, and 20%, respectively. Theother portion of the sample was twisted at room. temperature andsubsequently annealed at 300 C. for thirty minutes. This is one form ofa known low temperature strain tempering method and was carried out inorder to provide a reference. In no case was a dimensionchangingdeformation observed.

The change in Vickers hardness for 4% surface strain is illustrated inFIG. 3 with respect to the temperature of warm-working. Considering thatthe hardness as annealed was 62 and the hardness as cold-worked was 72,the effect of warm-working is seen to be appreciable at a temperature aslow as 100 C., reaching the maximum at 300 C., but falling beyond 400 C.However, above a temperature of 500 C., for example, the hardness dropsrapidly, as recrystallization commences.

The same observations were made for other surface strains, in additionto that of 4%, except that the hardness curves were shifted upwardly toa small extent with increasing strain at any given temperature. Theelfect was achieved with strains as small as 1%, although the incrementwas not as great as 4%.

In the case of cold-working followed by annealing at 300 0., namely theknown low temperature strain annealing referred to above, there was alsoan increase in hardness, but only to 80.

Therefore, the proper temperature range for warmworking of the methodaccording to the invention is defined as approximately between 200 C.and 400 C., the preferred range being in the neighborhood of 300 C.

The exact mechanism by which this phenomenon is achieved is not, as yet,completely known, but it is be lieved certain that the interaction ofdislocations with interstitial atoms, in the above-illustrated case,carbon atoms, and among themselves are responsible. In fact, in a studycarried out by transmission electron microscopy, it was found that thedensity of dislocations was unusually high for a given strain ascompared with coldworked or strain-tempered material, and that thosedislocations were effectively immobilized in mutual entanglement or bynewly formed precipitates of submicron sizes. Besides these, suchatomic, and therefore unobservable, processes as Cottrell atmosphereformation or interaction with vacancies and voids were believed to havecontributed. The rise and fall of the curve, and hence the presence ofthe optimum conditions, is probably the reflection of the balancebetween diffusion of the interstitial atoms and their reaction withdislocations.

Inasmuch as the present invention has been found to be readily practisedin conjunction with such apparently non-deforming processes astwist-straightening or spinfinishing, as often employed in modernmanufacturing as part of a predetermined sequence of operations, themethod of the invention can be integrated into industrialmass-production without any difficulty, provided that the aboveconditions are observed.

EXAMPLE 3 A steel pipe having a thickness of 2 mm. and the compositionlisted below in Table 2, while cooling from the temperature of hot rollforming in still air, was fed to a roller levelling machine at atemperature of about 300 C. There was a slight, though insignificant,reduction in thickness of less than 1%. In addition to achieving thedesired straightness, some 10 kg./mm. was gained in the tensile strengthas compared to samples finished in the customary manner, namely duringcompletion of the levelling at a temperature above the recrystallizationtemperature.

TABLE 2 Element, percent:

C 0.15 Si 0.35 Mn 1.06 P 0.018 N 0.008 S 0.025

While it is not known how much of the 0.15% total carbon was left asinterstitial at the temperature of the warm-working or how much wasrendered inefiicacious by forming stable carbide during cooling to thosetemperatures, practically the entire nitrogen must have existed as theinterstitial solute, since there was no stable nitrideforming element,such as aluminum or vanadium. Therefore, the very notable increase inthe strength can be considered as caused by interstitial carbon andinterstitial nitrogen. Thus, the control factors of the method of theinvention are temperature of the Warm-working of approximately 200 C. toapproximately 400 C., preferably in the neighborhood of 300 C., whetherthese temperatures are reached from below, namely by heating, or fromabove, namely by cooling, being immaterial, and the presence ofinterstitial solute atoms, of which as little as 0.02% carbon or 0.008%nitrogen were shown to be sufficient, and in addition the use of amethod of working where the surface strain of at least 1% Was achievedby twisting or repeated slight bending during roller levelling.

Furthermore, it will be understood that the present invention isapplicable to metals and alloys other than iron and steel, as long asthe basic principle of strengthening according to the method of theinvention, namely that due to the interaction of dislocations withinterstitial solute atoms, is universal.

Finally, it will be appreciated that the method of the present inventionis basically distinct from known method of warm-working in that whilethe latter necessarily give rise to a substantial dimensionaldeformation, such as a reduction in cross section, no such complexitiesare involved with the method of the invention. Moreover, the method'ofthe invention is not the same as, or even similar to, the known methodof low temperature strain tempering, as the straining and heating is notseparate with the method of the invention, while the straining andheating are separate in the known method.

Having thus described the principles and some applications of the methodof the invention, it will be immediately evident to those skilled in theart that the method of the invention is not limited to the few specificmethods which were discussed above by way of example and theillustrations of the drawings, except as limited by the appended claims.

What is claimed is:

1. Method of improving mechanical and physical properties of alpha ironsteel bar and rod articles of circular cross-section in both surface andinner layers thereof, which comprises subjecting such steel articlecontaining interstitial solute atoms of an element selected from thegroup consisting of carbon and nitrogen in an amount of at least 0.02%carbon and at least 0.008% nitrogen to warm working by twisting with asurface strain of at least 1% at a temperature between about 200 C. andabout 400 C., the plastic deformation by said twisting being inproportion to the cross sectional dimensions of said article, wherebythe mechanical and physical properties of said article are improvedwithout dimensional deformation thereof.

2. Method according to claim 1 and wherein said plastic deformation bytwisting is carried out on said steel article while the same is coolingfrom previous hot working References Cited themf- UN D STATES PATENTS 3.Method according to claim 1 wherein a hot drawn ITE metal article issubjected to said plastic deformation by $33 2? twisting.

4. Method according to claim 1 wherein said plastic 5 3,102,060 8/1963Schoch at 14812 deformation by twisting is accompanied by warm bend-3,196,052 7/1965 ing of said article.

5. Method according to claim 2 wherein said plastic HYLAND BIZOT PnmaryExammer deformation by twisting is accompanied by warm straight- 10 W.W. STALLARD, Assistant Examiner ening of said article.

